The present invention relates to a method for stacking and interconnecting segments of silicon, and more particularly to a method for stacking segments, which include a plurality of die and beveled edge walls, and interconnecting the segments on the edges of the stack using electrically conductive epoxy.
For many years, electrical components such as transistors and integrated circuits have been made using wafers of semiconductor material, including silicon and germanium. Integrated circuits have been provided on the wafer using various techniques known as etching, doping, and layering. Individual integrated circuits that are provided on the wafer are referred to as die, and include contact points called bond pads for external electrical connections. Typically, the die on the wafer are separated from one another by cutting the wafer along boundaries defining the die. Once the die are cut from the wafer, they are referred to as chips, and are packaged for use. In recent years, the proliferation of more powerful electronic systems has led to an increased need for higher density integrated circuit packages.
One method for creating higher density packages attempts to create entire computer systems on a single wafer using wafer scale integration (WSI) techniques. WSI technology attempts to laterally wire together all the die on a wafer using wires to interconnect the die. However, in order to create the necessary interconnections between the die, many wires are required that are extremely thin and difficult to create.
A second method for creating higher density packages attempts to reduce the area required for placing the chips on a circuit board by physically stacking the chips vertically. One chip stacking technique mounts individual die on ceramic carriers, encapsulates both the die and the carrier, stacks the carriers, and then mounts the stack on a printed circuit board. In this technique, all the die in the stack are interconnected by connecting the leads of the die to the printed circuit board via metal pins. This method results in an unusually high pin count on the circuit board which reduces the reliability of the circuitry because the high pin count increases the possibility that one of the many pins may become disconnected from the board.
Another chip stacking method uses a more complex process to stack die, as disclosed in U.S. Pat. No. 5,104,820 issued Apr. 14, 1992. As shown in FIG. 1, this method modifies individual chips 10 so that they may be stacked by adding a pattern of metallization, called rerouting leads 12, to the surface of the wafer. The rerouting leads 12 extend from the bond pads 14 on the chip 10 to newly formed bond pads 11, and are arranged so that all the rerouting leads 12 terminate on one side of the modified chip 10. Each modified chip 10 is then cut from the wafer, as shown by the dotted lines, and assembled into a stack (not shown). The stack is assembled in a manner such that all the leads 12 of the modified chips 10 are aligned along the same side of the stack. The side of the stack having the leads 12 is then etched and polished so that a cross section of the leads 12 on each of the modified chips 12 is accessible. After the leads 12 are exposed, a layer of metallization is applied to the leads 12 along the side of the stack in order to electrically connect each of the modified chips 10 in the stack. The stack is then mounted and connected to a substrate which in turn is connected to conventional circuitry.
The method of rerouting leads offers improvement in circuit density over prior methods but is complex and expensive. In addition, as shown in FIG. 1, the rerouting leads 12 extend over five adjacent die 15 through 19, which are destroyed when the modified chip 10 is cut out of the wafer. In this method, five die are sacrificed for every chip 10 that is modified.
Another method for creating higher density circuits creates stacks from entire wafers, rather than individual chips, to form a wafer array. In some devices, the wafers in the stack are electrically interconnected using solid vertical columns of metallic conductive feed-throughs, such as copper. The use of solid feed-throughs to interconnect wafers may cause damage to the array due to differential thermal coefficients of expansion during thermal cycles. Furthermore, the process is costly and makes the wafers difficult to separate for repairs.
Other methods also exist to interconnect stacks of wafers, as disclosed in, for example, U.S. Pat. No. 4,897,708 issued Jun. 30, 1990, and U.S. Pat. No. 4,954,875 issued Sep. 4, 1990. These methods provide each wafer in the stack with coned-shaped through holes which expose bonding pads on the wafers. The bond pads of the wafers in the stack are then electrically connected by either filling the through holes with electrically conductive liquid, or inserting an electrically conductive compliant material into the through holes, to provide a continuous vertical electrical connection between the wafers. While avoiding the disadvantages of using solid vertical columns of metal to interconnect wafers, the use of electrically conductive liquids and conductive materials requires special tooling to fill the through holes. Furthermore, for some applications, it may not be desirable to use stacks of entire wafers due to size constraints of the electrical device.